Polypeptide porous body and method for producing same

ABSTRACT

A polypeptide porous body of the present invention is a porous body of a polypeptide derived from spider silk proteins. The polypeptide includes a water-insoluble polypeptide. The polypeptide porous body has an apparent density of 0.1 g/cm3 or less. A method for producing the polypeptide porous body includes: a solution production step in which the polypeptide is dissolved in at least one solvent selected from DMSO, DMF, and these with an inorganic salt, so as to obtain a solution of the polypeptide; a step in which the solution produced in the solution production step is substituted with a water-soluble solvent so as to obtain a polypeptide gel; and a step in which the polypeptide gel is dried. Thereby, the present invention provides a polypeptide porous body having excellent water absorbability and a polypeptide porous body suitable for application to a living body, and a method for producing the same.

TECHNICAL FIELD

The present invention relates to a polypeptide porous body made from a polypeptide derived from spider silk proteins, and a method for producing the same.

BACKGROUND ART

A polypeptide hydrogel is used as a biomaterial for artificial cartilage, etc. Patent Document 1 proposes a hydrogel, a foam, and the like obtained by dissolving silk fibroin in a hygroscopic polymer such as polyethylene glycol. Patent Document 2 discloses a photocrosslinked gel made from spider silk.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 2007-515391 A

Patent Document 2: JP 2008-506409 A

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, an urea aqueous solution, which is a conventionally proposed solvent, does not have a sufficient power to dissolve spider silk proteins, and a guanidine aqueous solution, hexafluoroisopropanol (HFIP), and the like are expensive and difficult to be applied to a living body if they remain in a product. Further, the water absorbability of conventional silk fibroin foams is unknown.

To solve the above conventional problems, the present invention provides a polypeptide porous body having excellent water absorbability and a polypeptide porous body suitable for application to a living body, and a method for producing the same.

Means for Solving Problem

A polypeptide porous body of the present invention is a porous body of a polypeptide derived from spider silk proteins, wherein the polypeptide includes a water-insoluble polypeptide, and the polypeptide porous body has an apparent density of 0.1 g/cm³ or less.

A method for producing a polypeptide porous body of the present invention includes: a solution production step in which a polypeptide derived from spider silk proteins is dissolved in at least one dissolving solvent selected from the group consisting of: (A) dimethyl sulfoxide; (B) dimethyl sulfoxide with an inorganic salt; and (C) N, N-dimethylformamide with an inorganic salt, so as to obtain a solution of the polypeptide; a step in which the solution of the polypeptide produced in the solution production step is substituted with a water-soluble solvent so as to obtain a polypeptide gel; and a step in which the polypeptide gel is dried. Incidentally, “a polypeptide is dissolved in a dissolving solvent” used herein includes both of a state in which a polypeptide is completely dissolved in a dissolving solvent and a state in which polypeptide microparticles are dispersed in a dissolving solvent and thus are substantially dissolved in the dissolving solvent. Hereinafter, the expression shall have the same meaning as described above.

Effect of the Invention

In the present invention, by using a specific solvent in the production of the polypeptide solution, substituting the solution with a water-soluble solvent to obtain a polypeptide gel, and drying the obtained gel, a polypeptide porous body can be produced in which the amount of the remaining solvent is little or the amount of the remaining solution is sufficiently low, whereby a polypeptide porous body suitable for application to a living body can be provided. Further, since the apparent density of the polypeptide porous body according to the present invention is sufficiently low, a void volume inside the porous body is large. Thus, the polypeptide porous body exhibits high water absorbability. The porous body is also called as a xerogel, which has a mesh structure having voids in a solid phase skeleton. Additionally, the solvents used in the present invention are those that have been used in the production of acrylic fibers and polyimid, and they are low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical photograph of a polypeptide porous body in one example of the present invention.

FIG. 2 is a photograph of the polypeptide porous body in the example of the present invention taken by a scanning electron microscope (SEM) at 70× magnification.

FIG. 3 is a photograph of the same at 200× magnification.

FIG. 4 is a photograph of the same at 1,000× magnification.

DESCRIPTION OF THE INVENTION

A polypeptide derived from spider silk proteins is used as the protein of the present invention. The polypeptide derived from spider silk proteins is not limited particularly as long as it is derived from or similar to natural type spider silk proteins. Examples of the polypeptide derived from spider silk proteins include variants, analogs, derivatives, and the like of natural type spider silk proteins. In terms of excellent tenacity, the recombinant spider silk protein preferably is a recombinant spider silk protein derived from major dragline silk proteins produced in major ampullate glands of spiders. Examples of the major dragline silk proteins include major ampullate spidroins MaSp1 and MaSp2 derived from Nephila clavipes, and ADF3 and ADF4 derived from Araneus diadematus, etc.

The recombinant spider silk protein may be a recombinant spider silk protein derived from minor dragline silk produced in minor ampullate glands of spiders. Examples of the minor dragline silk proteins include minor ampullate spidroins MiSp1 and MiSp2 derived from Nephila clavipes.

Other than these, the recombinant spider silk protein may be a recombinant spider silk protein derived from flagelliform silk proteins produced in flagelliform glands of spiders. Examples of the flagelliform silk proteins include flagelliform silk proteins derived from Nephila clavipes, etc.

Examples of the polypeptide derived from major dragline silk proteins include a polypeptide containing two or more units of an amino acid sequence represented by the formula 1: REP1−REP2 (1), preferably a polypeptide containing four or more units thereof, and more preferably a polypeptide containing six or more units thereof. In the polypeptide derived from major dragline silk proteins, units of the amino acid sequence represented by the formula (1): REP1−REP2 (1) may be the same or different from each other. In the formula (1), the REP1 represents polyalanine. In the REP1, the number of alanine residues arranged in succession is preferably 2 or more, more preferably 3 or more, further preferably 4 or more, and particularly preferably 5 or more. Further, in the REP1, the number of alanine residues arranged in succession is preferably 20 or less, more preferably 16 or less, further preferably 14 or less, and particularly preferably 12 or less. In the formula (1), the REP2 is an amino acid sequence composed of 10 to 200 amino acid residues. The total number of glycine, serine, glutamine, proline, and alanine residues contained in the amino acid sequence is 40% or more, preferably 50% or more, and more preferably 60% or more with respect to the total number of amino acid residues contained therein.

In the major dragline silk, the REP1 corresponds to a crystal region in a fiber where a crystal β sheet is formed, and the REP2 corresponds to an amorphous region in a fiber where most of the parts lack regular structures and that has more flexibility. Further, the [REP1−REP2] corresponds to a repetitious region (repetitive sequence) composed of the crystal region and the amorphous region, which is a characteristic sequence of dragline silk proteins.

An example of the polypeptide containing two or more units of the amino acid sequence represented by the formula 1: REP1−REP2 (1) is a recombinant spider silk protein derived from ADF3 having an amino acid sequence represented by any of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4. The amino acid sequence represented by SEQ ID NO: 1 is an amino acid sequence from the 1st residue to the 631st residue of an amino acid sequence that is obtained by adding an amino acid sequence (SEQ ID NO: 5) composed of a start codon, an His 10-tag and an HRV3C Protease (Human rhinovirus 3C Protease) recognition site to the N-terminal of a partial amino acid sequence of ADF3 obtained from the NCBI database (NCBI Genebank Accession No.: AAC47010, GI: 1263287). The amino acid sequence represented by SEQ ID NO: 2 is an amino acid sequence obtained by the following mutation: in an amino acid sequence of ADF3 (NCBI Genebank Accession No.: AAC47010, GI: 1263287) to the N-terminal of which has been added the amino acid sequence (SEQ ID NO: 5) composed of a start codon, an His 10-tag and an HRV3C Protease (Human rhinovirus 3C Protease) recognition site, 1st to 13th repetitive regions are about doubled and the translation ends at the 1154th amino acid residue. The amino acid sequence represented by SEQ ID NO: 3 is an amino acid sequence obtained by adding the amino acid sequence (SEQ ID NO: 5) composed of a start codon, an His 10-tag and an HRV3C Protease (Human rhinovirus 3C Protease) recognition site, to the N-terminal of a partial amino acid sequence of ADF3 (NCBI Genebank Accession No.: AAC47010, GI: 1263287) obtained from the NCBI database. The amino acid sequence represented by SEQ ID NO: 4 is an amino acid sequence obtained as follows: in an amino acid sequence of ADF3 (NCBI Genebank Accession No.: AAC47010, GI: 1263287) to the N-terminal of which has been added the amino acid sequence (SEQ ID NO: 5) composed of a start codon, an His 10-tag and an HRV3C Protease (Human rhinovirus 3C Protease) recognition site, 1st to 13th repetitive regions are about doubled. Further, the polypeptide containing two or more units of the amino acid sequence represented by the formula 1: REP1−REP2 (1) may be a polypeptide that is composed of an amino acid sequence represented by any of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3 and SEQ ID NO: 4 in which one or a plurality of amino acids have been substituted, deleted, inserted and/or added and that has repetitious regions composed of crystal regions and amorphous regions.

In the present invention, “one or a plurality of” refers to 1 to 40, 1 to 35, 1 to 30, 1 to 25, 1 to 20, 1 to 15, 1 to 10, or 1 or a few, for example. Further, in the present invention, “one or a few” refers to 1 to 9, 1 to 8, 1 to 7, 1 to 6, 1 to 5, 1 to 4, 1 to 3, 1 to 2, or 1.

An example of the recombinant spider silk protein derived from minor dragline silk proteins is a polypeptide containing an amino acid sequence represented by the formula 2: REP3 (2). In the formula 2, the REP 3 indicates an amino acid sequence composed of (Gly-Gly-Z)m(Gly-Ala)l(A)r, where Z indicates any one of amino acids, in particular, it preferably is an amino acid selected from the group consisting of Ala, Tyr and Gln. Further, preferably, m is 1 to 4, 1 is 0 to 4, and r is 1 to 6.

Among spider silks, the minor dragline silk is wound spirally from the center of a spider net, and used as a reinforcement of the net and a yarn to wrap a captured prey. The minor dragline silk is inferior to the major dragline silk in tensile strength, but is known to have high stretchability. The reason for this is considered to be as follows: in the minor dragline silk, since many crystal regions are formed of regions where glycine and alanine are arranged alternately in succession, hydrogen bonds in the crystal regions weaken easily as compared with the major dragline silk whose crystal regions are formed only of alanine.

Examples of the recombinant spider silk protein derived from flagelliform silk proteins include a polypeptide containing an amino acid sequence represented by the formula 3: REP4 (3). In the formula 3, the REP 4 indicates an amino acid sequence composed of (Gly-Pro-Gly-Gly-X)n, where X indicates any one of amino acids, in particular, it preferably is an amino acid selected from the group consisting of Ala, Ser, Tyr and Val. Further, n indicates a number of 4 or larger, preferably 10 or larger, and more preferably 20 or larger.

Among spider silks, the flagelliform silk does not have crystal regions but has repetitious regions composed of amorphous regions, which is a major characteristic of the flagelliform silk. It is considered that since the major dragline silk and the like have repetitious regions composed of crystal regions and amorphous regions, they have both of high strength and stretchability. Meanwhile, regarding the flagelliform silk, the strength is inferior to that of the major dragline silk but the stretchability is high. The reason for this is considered to be that the flagelliform silk is composed mostly of amorphous regions.

The polypeptide can be produced using a host that has been transformed by an expression vector containing a gene encoding a polypeptide. A method for producing a gene is not limited particularly, and it may be produced by amplifying a gene encoding a natural type spider silk protein from a cell containing the desired gene of spider by a polymerase chain reaction (PCR), etc., and cloning it, or may be synthesized chemically. A method for chemically synthesizing a gene also is not limited particularly, and it can be synthesized as follows, for example: based on information of amino acid sequences of natural type spider silk proteins obtained from the NCBI web database, oligonucleotides that have been synthesized automatically with AKTA oligopilot plus 10/100 (GE Healthcare Japan Corporation) are linked by PCR, etc. At this time, in order to facilitate purification and observation of protein, a gene may be synthesized that encodes a protein composed of an amino acid sequence in which an amino acid sequence composed of a start codon and an His 10-tag has been added to the N-terminal of the amino acid sequence. Examples of the expression vector include a plasmid, a phage, a virus, and the like that can express protein based on a DNA sequence. The plasmid-type expression vector is not limited particularly as long as it allows a target gene to be expressed in a host cell and it can amplify itself. For example, in the case of using Escherichia coli Rosetta (DE3) as a host, a pET22b(+) plasmid vector, a pCold plasmid vector, and the like can be used. Among these, in terms of productivity of protein, the use of the pET22b(+) plasmid vector is preferred. Examples of the host include animal cells, plant cells, microbes, etc.

Further, as the polypeptide, a polypeptide that can form a gel upon contact with a water-soluble solvent is used. An example of the polypeptide that can form a gel is a polypeptide that aggregates easily (e.g., a polypeptide having a relatively large molecular weight) when the solution produced in the solution production step is substituted with a water-soluble solvent. Incidentally, even if a polypeptide does not have such an easy aggregation property, for example, by increasing the concentration of the polypeptide in the solution produced in the solution production step, the polypeptide aggregates when the solution is substituted with a water-soluble solvent. Thus, a polypeptide gel is formed. The polypeptide also may aggregate by lowering the temperature from a high-temperature state to a low-temperature state. In other words, a polypeptide porous body is produced using a polypeptide that aggregates and forms a gel when the solution produced in the solution production step is substituted with a water-soluble solvent. The polypeptide porous body includes such a polypeptide.

The polypeptide porous body has continuous pores. It is believed that the continuous pores are formed by through holes of water desorbed in the vacuum freeze-drying step. As a result, a xerogel is formed, which has a mesh structure having voids in a solid phase skeleton. Such a polypeptide porous body has an apparent density of 0.1 g/cm³ or less, preferably in a range from 0.01 to 0.08 g/cm³. When the apparent density is less than 0.01 g/cm³, the void volume will be excessive because the apparent density is too small, which may decrease the strength of the polypeptide porous body.

The maximum water absorption ratio of the porous body is preferably two times or more, and more preferably three times or more, in terms of weight. The upper limit thereof is preferably 20 times or less, and more preferably 18 times or less. The maximum water absorption ratio indicates a point at which the porous body absorbs water to the maximum and does not leak water by inclination.

The polypeptide porous body of the present invention is produced by: a solution production step in which a polypeptide derived from spider silk proteins is dissolved in at least one dissolving solvent selected from the group consisting of (A) dimethyl sulfoxide; (B) dimethyl sulfoxide with an inorganic salt; and (C) N, N-dimethylformamide with an inorganic salt, so as to obtain a solution of the polypeptide; a step in which the solution of the polypeptide produced in the solution production step is substituted with a water-soluble solvent so as to obtain a polypeptide gel; and a step in which the polypeptide gel is dried. The polypeptide porous body can be formed into a predetermined shape by performing a molding step in which the solution is poured into a mold between the solution production step and the solvent substitution step using a water-soluble solvent, or by cutting the gel after the solvent substitution step using a water-soluble solvent. Further, at least one selected from the group consisting of dimethyl sulfoxide and N, N-dimethylformamide may be present inside the obtained polypeptide porous body. The amount of the dissolving solvent present therein is not limited particularly, and it is an amount that remains therein unintentionally after the solution produced in the solution production step is substituted with the water-soluble solvent.

In addition to the substances indicated in (A)-(C) above, the dissolving solvent may contain alcohol and/or water. The dissolving solvent is a polar solvent, and tends to absorb moisture in air. Therefore, in some cases, commercial solvents contain several % of water. The dissolving solvent may contain this amount of water and/or alcohol. Incidentally, the substances functioning as the dissolving solvent are those indicated in (A)-(C) above.

The water-soluble solvent refers to a solvent containing water. Examples of the water-soluble solvent include water, a water-soluble buffer solution, and saline.

In terms of high compatibility with the human body, preferably, the water-soluble solvent is water. Although the water is not limited particularly, it may be pure water, distilled water, ultrapure water, etc.

The viscosity of the solution after the solution production step is preferably 5 to 80 cP (centipoises), and more preferably 10 to 50 cP. Within the above range, favorable handleability and convenience will be obtained.

In the present invention, a solvent containing DMSO and/or DMF (polar solvent) is used as the solvent. DMSO has a melting point of 18.4° C. and a boiling point of 189° C. DMF has a melting point of −61° C. and a boiling point of 153° C. DMSO and DMF have much higher boiling points than hexafluoroisopropanol (HFIP) and hexafluoroacetone (HFAc) having boiling points of 59° C. and −26.5° C., respectively, which have been used in conventional methods, and hence DMSO and DMF have favorable dissolubility. Further, in view of the fact that DMSO and DMF have been used also in general industrial fields for acrylic fiber polymerization, acrylic fiber spinning solutions, and solvents for polyimide polymerization, they are low-cost substances with proven safety.

Addition of an inorganic salt to DMSO or DMF further increases the solubility of a solute. The inorganic salt is at least one selected from alkali metal halides (e.g., LiCl, LiBr, etc), alkaline-earth metal halides (e.g., CaCl₂), alkaline-earth metal nitrate (e.g., Ca(NO₃)₂, etc.), and sodium thiocyanate (e.g., NaSCN, etc.). When the dissolved components are assumed to be 100 mass %, the percentage of the inorganic salt preferably ranges from 0.1 to 20 mass %.

The polypeptide gel is produced by substituting the solvent with the water-soluble solvent. At this time, the polypeptide gel is not dissolved in water. Preferably, the step of substituting the solvent with the water-soluble solvent is performed in the following manner: the solution of the polypeptide obtained by dissolving the polypeptide in the solvent is placed in a dialysis membrane, the dialysis membrane is immersed in a water-soluble solvent, and the water-soluble solvent is renewed at least one time. Specifically, preferably, the step of substituting the solvent with the water-soluble solvent is performed by placing the solution after the solution production step in a dialysis membrane, leaving it to stand for 3 hours in a water-soluble solvent in an amount 100 times or more the amount of the solution (one batch), and renewing the water-soluble solvent three or more times in total. Any dialysis membrane that does not allow the polypeptide in the solution to pass therethrough can be used. An example of the dialysis membrane is a cellulose dialysis membrane. By repeating the substitution using the water-soluble solvent, the amount of the dissolving solvent can be reduced close to zero. In the latter half of the desolvation step, it is unnecessary to use a dialysis membrane.

The amount of the solvent, i.e., dimethyl sulfoxide (DMSO) or N, N-dimethylformamide (DMF), remaining in the polypeptide after the substitution step using the water-soluble solvent can be measured by a nuclear magnetic resonance spectrometer (NMR). A 1,2-dichloroethane-formic acid solution can be used as an internal standard.

In the drying step, preferably, vacuum freeze-drying is adopted. The degree of vacuum at vacuum freeze-drying is preferably 200 Pa or less, more preferably 150 Pa or less, and further preferably 100 Pa or less. By vacuum drying, water evaporates from the polypeptide gel, and the temperature declines by the evaporation latent heat, whereby it is brought into a frozen state. The temperature of the polypeptide at vacuum freeze-drying is preferably 70° C. or less, more preferably 60° C. or less, and further preferably 50° C. or less. Incidentally, prior to vacuum freeze-drying, the polypeptide gel may be pre-frozen at a temperature of −10° C. to −45° C. for about 10 to 36 hours. The moisture content after freeze-drying is preferably 5.0% or less, and more preferably 3.0% or less.

EXAMPLES

Hereinafter, the present invention will be described in further detail by way of examples. Note that the present invention is not limited to the following examples. Water is used as the water-soluble solvent in the examples.

<Methods of Various Measurements>

(1) Measurement of Remaining Amount of Solvent

As an internal standard, a 1,2-dichloroethane-formic acid solution at a concentration of 3,100 ppm (0.00310 mg/ml) was prepared. 500 μl of a protein solution (obtained by dissolving 0.1 g of a polypeptide porous body in 10 ml of formic acid) and 500 μl of an internal standard solution were mixed. For H-NMR measurement, an acetonitrile deuterated solvent was added to the mixed solution in an amount approximately equivalent to that of the mixture solution so as to dilute the solution to about two times. Then, H-NMR measurement was performed (NMR model: JNM-ECX 100 manufactured by JOEL Ltd.). The H-NMR integrated intensity of 1,2-dichloroethane (internal standard sample) was compared with the H-NMR integrated intensity of DMSO. A calibration curve was formed by preparing a DMSO-formic acid solution at 3 ppm to 3000 ppm and following the above-mentioned protocol. By comparison with the calibration curve, the concentration of DMSO in the protein solution was calculated. A nuclear magnetic resonator (NMR) manufactured by JOEL Ltd. was used for the measurement of the concentration of DMSO.

(2) Viscosity

An EMS machine manufactured by Kyoto Electronics Manufacturing Co., Ltd. was used.

Example 1

1. Preparation of Polypeptide

<Gene Synthesis of ADF3Kai-A>

A partial amino acid sequence of ADF3, which is one of two principal dragline silk proteins of Araneus diadematus, was obtained from the NCBI web database (NCBI Accession No.: AAC47010, GI: 1263287), and an amino acid sequence (SEQ ID NO: 5) composed of a start codon, an His 10-tag and an HRV3C Protease (Human rhinovirus 3C Protease) recognition site was added to the N-terminal of the partial amino acid sequence of ADF3, so as to synthesize a gene encoding a polypeptide (ADF3Kai-A) composed of an amino acid sequence (SEQ ID NO: 1), i.e., the 1st residue to the 631st residue from the N-terminal of the resultant sequence. Consequently, a pUC57 vector to which a gene of ADF3Kai-A composed of a base sequence represented by SEQ ID NO: 6 had been introduced was obtained (having an Nde I site immediately upstream of the 5′ terminal of the gene and an Xba I site immediately downstream of the 5′ terminal thereof). Thereafter, the gene was subjected to a restriction enzyme treatment with Nde I and EcoR I, and recombined into a pET22b(+) expression vector. Thus, a pET22b(+) vector to which the gene of ADF3Kai-A had been introduced was obtained.

<Expression of Protein>

The obtained pET22b(+) expression vector containing the gene sequence of ADF3Kai-A was transformed into Escherichia coli Rosetta (DE3). The obtained single colony was incubated for 15 hours in 2 mL of an LB culture medium containing ampicillin. Thereafter, 1.4 ml of said culture solution was added to 140 mL of an LB culture medium containing ampicillin, and incubated to an OD₆₀₀ of 3.5 under the conditions of 37° C. and 200 rpm. Next, the culture solution with the OD₆₀₀ of 3.5 was added to 7 L of a 2×YT culture medium containing ampicillin, together with 140 mL of 50% glucose, and incubated further to the OD₆₀₀ of 4.0. Thereafter, isopropyl-β-thiogalactopyranoside (IPTG) was added to the obtained culture solution with the OD₆₀₀ of 4.0 so that the fmal concentration would be 0.5 mM, thereby inducing the expression of protein. After a lapse of two hours from the addition of IPTG, the culture solution was centrifuged and bacterial cells were collected. Protein solutions prepared from the culture solution before the addition of IPTG and the culture solution after the addition of IPTG were each electrophoresed in a polyacrylamide gel. Consequently, a target band size (about 56.1 kDa) was observed with the addition of IPTG, and the expression of the target protein (ADF3Kai-A) was confirmed.

Purification

(1) About 50 g of bacteria cells of the Escherichia coli expressing the ADF3Kai-A protein and 300 ml of a buffer solution M (20 mM Tris-HCl, pH 7.4) were placed in a centrifuge tube (1000 ml). After dispersing the bacteria cells with a mixer (“T18 basic ULTRA TURRAX” manufactured by IKA, level 2), the dispersion was centrifuged (11,000 g, 10 minutes, room temperature) with a centrifuge (“Model 7000” manufactured by Kubota Corporation), and a supernatant was discarded.

(2) To a precipitate (bacteria cells) obtained by the centrifugation, 300 ml of the buffer solution M and 3 ml of 0.1 M PMSF (dissolved by isopropanol) were added. After dispersing the precipitate for 3 minutes with the above mixer (level 2) manufactured by IKA, the bacteria cells were disrupted repeatedly for three times using a high-pressure homogenizer (“Panda Plus 2000” manufactured by GEA Niro Soavi).

(3) To the disrupted bacterial cells, 300 mL of a buffer solution B (50 mM Tris-HCL, 100 mM NaCl, pH 7.0) containing 3 w/v % of SDS was added. After dispersing well the bacterial cells with the above mixer (level 2) manufactured by IKA, the dispersion was stirred for 60 minutes with a shaker (manufactured by TAITEC CORPORATION, 200 rpm, 37° C.). Thereafter, the stirred dispersion was centrifuged (11,000 g, 30 minutes, room temperature) with the above centrifuge manufactured by Kubota Corporation, and a supernatant was discarded, whereby SDS washing granules (precipitate) were obtained.

(4) The SDS washing granules were suspended in a DMSO solution containing 1M lithium chloride so that the concentration would be 100 mg/mL, and heat-treated for 1 hour at 80° C. Thereafter, the heated suspension was centrifuged (11,000 g, 30 minutes, room temperature) with the above centrifuge manufactured by Kubota Corporation, and a supernatant was collected.

(5) Ethanol in an amount three times greater than that of the collected supernatant was prepared. The collected supernatant was added to the ethanol, and left to stand for 1 hour at room temperature. Thereafter, the resultant was centrifuged (11,000 g, 30 minutes, room temperature) with the above centrifuge manufactured by Kubota Corporation to collect aggregated protein. Next, a process of washing aggregated protein using pure water and collecting aggregated protein by centrifugation was repeated three times, and thereafter water was removed by a freeze dryer to collect freeze-dried powder. The purification degree of the target protein ADF3Kai-A (about 56.1 kDa) in the obtained freeze-dried powder was checked by analyzing images of the results of polyacrylamide gel electrophoresis (CBB staining) of said protein powder using Totallab (Nonlinear Dynamics). As a result, the purification degree of ADF3Kai-A was about 85%.

2. Adjustment of Solution

0.8 g of spider silk protein powder (ADF3Kai-A) was added to 20 ml of DMSO (containing 1M LiCl), followed by dissolution at 80° C. for 30 minutes. Then, dusts and bubbles were removed from the solution. The viscosity of the solution was 30.8 cP (centipoises). The solution was placed a dialysis tube (Cellulose Tube 36/32 manufactured by Sanko Junyaku Co., Ltd. (presently EIDIA Co., Ltd)).

3. Substitution with Water

The dialysis tube was placed in a beaker filled with 3 L of pure water, left to stand for 3 hours, and water was renewed. This operation was repeated six times in total. Thus, the spider silk protein in the solution aggregated, and a hydrogel in which almost all of DMSO was substituted with water was produced. The moisture content of the obtained gel was 95.3 mass %.

4. Vacuum Freeze-drying

The hydrogel was freeze-dried by a freeze dryer (“FDU-1200” manufactured by Tokyo Rikakiki Co., Ltd.) under conditions of 14 Pa and −45° C. for 15 hours.

5. Results

(1) The amount of the solvent remaining in the obtained porous body was 2.63 g based on 100 g of the porous body.

(2) The apparent density of the obtained porous body was 0.077 g/cm³. Incidentally, the apparent density was measured in accordance with a known technique. The densities (apparent densities) of Examples 2-4 described below also were measured in the same manner as in Example 1.

(3) The maximum water absorption ratio of the obtained porous body was 15.4 times. This water absorption ratio indicates a point at which the porous body absorbs water to the maximum and does not leak water by inclination. The same applies to the following examples.

(4) FIGS. 1-4 show the obtained porous body.

Example 2

A gel of Example 2 was produced in the same manner as in Example 1 using the same polypeptide as that of Example 1. A 20 mg/ml dope (DMSO+1M LiCl) was prepared using 0.4 g of the polypeptide powder so as to produce a gel with a low concentration. The viscosity of the dope was 13.9 cP. The moisture content of the obtained gel was 98.8 mass %. The gel was subjected to vacuum freeze-drying under the same condition as in Example 1. The density of the obtained porous body was 0.020 g/cm³. The maximum water absorption ratio of the obtained porous body was 7.2 times.

Example 3

A gel of Example 3 was produced in the same manner as in Example 1 using the same polypeptide as that of Example 1. A 20 mg/ml dope (DMSO without salt) was prepared using 0.4 g of the polypeptide powder so as to produce a gel. The moisture content of the obtained gel was 97.4 mass %. The gel was subjected to vacuum freeze-drying under the same condition as in Example 1. The density of the obtained porous body was 0.036 g/cm³. The maximum water absorption ratio of the obtained porous body was 6.1 times.

Example 4

A gel of Example 4 was produced in the same manner as in Example 1 using the same polypeptide as that of Example 1. A 20 mg/ml dope (DMF+1M LiCl) was prepared using 0.4 g of the polypeptide powder so as to produce a gel. The moisture content of the obtained gel was 98.2 mass %. The gel was subjected to vacuum freeze-drying under the same condition as in Example 1. The density of the obtained porous body was 0.039 g/cm³. The maximum water absorption ratio of the obtained porous body was 3.8 times.

Although water was used in the substitution step in the above Examples 1-4, the same effect can be obtained using other water-soluble solvents.

INDUSTRIAL APPLICABILITY

The polypeptide porous body of the present invention is useful as a slow-release substrate such as a medicament and perfume, an absorber, etc.

SEQUENCE LISTING FREE TEXT

SEQ ID NOS: 1-5 amino acid sequences

SEQ ID NO: 6 base sequence 

The invention claimed is:
 1. A polypeptide porous body that is a porous body of a polypeptide derived from spider silk proteins, wherein the polypeptide includes a water-insoluble polypeptide derived from spider silk proteins, the polypeptide porous body has an apparent density of 0.1 g/cm³ or less, and at least one selected from the group consisting of dimethyl sulfoxide and N, N-dimethylformamide is present inside the polypeptide porous body.
 2. The polypeptide porous body according to claim 1, wherein the polypeptide porous body has an apparent density of 0.01 to 0.08 g/cm³.
 3. The polypeptide porous body according to claim 1, wherein the polypeptide porous body has a water absorption ratio of 2 to 20 times in terms of weight of the polypeptide porous body.
 4. A method for producing a polypeptide porous body, comprising: a solution production step in which a polypeptide derived from spider silk proteins is dissolved in at least one dissolving solvent selected from the group consisting of: (A) dimethyl sulfoxide; (B) dimethyl sulfoxide with an inorganic salt; and (C) N, N-dimethylformamide with an inorganic salt, so as to obtain a solution of the polypeptide; a step in which the solution of the polypeptide produced in the solution production step is substituted with a water-soluble solvent so as to obtain a polypeptide gel; and a step in which the polypeptide gel is dried.
 5. The method for producing a polypeptide porous body according to claim 4, wherein the polypeptide porous body is produced using an aqueous solution that is obtained by substituting the solution of the polypeptide with water.
 6. The method for producing a polypeptide porous body according to claim 4, wherein the substitution step using the water-soluble solvent is a step in which the solution of the polypeptide obtained by dissolving the polypeptide in the dissolving solvent is placed in a dialysis membrane, the dialysis membrane is immersed in a water-soluble solvent, and the water-soluble solvent is renewed at least one time.
 7. The method for producing a polypeptide porous body according to claim 4, wherein the drying is vacuum freeze-drying. 